The invention relates to a plunger switch for use as a switch element in an electrical circuit, having a housing and a plunger that is able to move axially in a bore of the housing in the direction of a longitudinal axis and when in an operating position in which it is depressed toward the housing, acts on an electronic sensor unit situated in the housing, which in turn initiates a switching procedure in the electrical circuit by means of electrical pulses.
A plunger switch of this kind is known, for example, from EP 1 710 821 A1 or US 2007/0290642 A1.
U.S. Pat. No. 4,481,806 A describes a plunger switch that acts on an inductive proximity switch with an oscillating electromagnetic field, which is interrupted in order to initiate a switching procedure by means of a metal pin.
U.S. Pat. No. 4,476,359 A discloses a push-button with a helical compression spring for mechanically driving a switch element.
Plunger switches are generally used to initiate switching procedures in electrical circuits by means of mechanical actuation of the plunger, in particular in order to close and/or open electrical switching contacts, which then in turn can trigger control operations in a larger system of which it is a part.
The vast majority of plunger switches currently available on the market are still based on a purely mechanical contact actuation of the type described, for example, in US 2008/0258853 A1. The plunger switches disclosed in US 2009/0127080 A1 or U.S. Pat. No. 4,638,276 A likewise have mechanical contact elements and each feature a lubrication of the plunger.
As discussed at the beginning, however, electronic plunger switches are also already available in which only the triggering of the switching procedure is produced by the mechanical movement of the plunger; the actual physical opening and closing of electrical contacts is carried out by means of a sensor unit that reacts to the plunger movement and can be embodied in a wide variety of ways. It is thus possible to simply detect axial on/off positions of the plunger without requiring special adaptation or design of the plunger and electrical contacts with regard to closing force and the like. In addition, this permits an automated electronic state diagnosis or function check of the plunger switch to be carried out in a simple way. Frequently, switches of this kind are also used in vehicle transmissions for detecting the current shifting state.
As before, though, even when using electronic sensor units in plunger switches, because of the necessary mechanical action of the plunger, which does in fact have to be actuated either automatically by moving machine parts or manually by an operator, the best that can be achieved is a switching capacity of at most 1 million actuations over the service life of the plunger switch. After this, at the very least, the mechanical part of the switch is worn out and as a rule, the entire plunger switch must be replaced.
In the plunger switch described in the above-mentioned US 2009/0127080 A1, whose plunger acts on a purely mechanical switch element, in order to separate an actuating chamber from a contact chamber, a diaphragm and other moving parts are provided, which result in high friction-induced actuation forces at various temperatures. Experience has shown that such an apparatus can malfunction after a few hundred thousand actuations.